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BIOLOGY  
 
F215:  Control,  Genomes  and  Environment  
1) Cellular  Control  and  Variation  
a) Cellular  Control  
i) State  that  genes  code  for  polypeptides,  including  enzymes  
ii) Explain  the  meaning  of  the  term  genetic  code  
iii) Describe,  with  the  aid  of  diagrams,  the  way  in  which  a  nucleotide  sequence  codes  for  the  
amino  acid  sequence  in  a  polypeptide  
(1) Protein  –  a  large  polypeptide,  usually  composed  of  100  or  more  amino  acids  and  one  
or  more  polypeptide  chains  
(2) Polypeptide  –  polymer  of  a  specific  sequence  of  amino  acids  joined  by  peptide  bonds  
(3) Allele   –   alternative   version   of   a   gene,   still   at   the   same   locus   on   the   chromosome   but  
the  alteration  to  the  DNA  base  sequence  may  alter  the  proteins  structure  
(4) DNA   –   a   polynucleotide   or   a   long   sequence   of   DNA   nucleotides   (genetic   material)  
that  codes  for  protein  synthesis/a  sequence  of  amino  acids  in  a  polypeptide  
(5) Gene   –   short   section   of   a   length   of   DNA   with   up   to   2000   in   one   chromosome   that  
codes  for  one  or  more  polypeptides  (a  unit  of  heredity)  eg…  
(a) Structural  proteins  eg
...
  TCT   codes   for   serine   in   any   organism   but   in  
ciliated  protoctists,  two  of  the  standard  stop  codes  code  for  glutamic  acid  
(8) Genome  (of  an  organism)  –  the  entire  DNA  sequence  of  that  organism  

(9) Nucleotide  –  monomer  of  all  nucleic  acids  

(a) Pentose  sugar  (C₅H₁₀O₄)  –  either  ribose  in  RNA  or  deoxyribose  in  DNA  
(i) Forms   covalent   bonds   with   the   nitrogenous   base   and   the   phosphate   groups  
via  condensation  reactions  
(ii) Nitrogenous  base  bonds  with  hydroxyl  group  on  first  carbon  
(iii) Phosphate  group  bonds  with  hydroxyl  group  on  fifth  carbon  
(b) Organic  nitrogenous  bases  (at  right  angles  to  the  sugars)  
(i) Purines  –  double  rings  of  carbon  and  nitrogen,  adenine  and  guanine  
(ii) Pyrimidines  –  single  rings  of  carbon  and  nitrogen  (fairly  stable  as  it  is  a  single  
molecule),  thymine  cytosine  and  uracil  
(c) Phosphoric  acid  (inorganic  molecule  abbreviated  to  Pi)  
(i) Negatively  charged  as  the  hydrogens  in  the  hydroxyl  groups  are  lost  
(ii) Molecule  attracts  water  forming  a  protective  coat  around  the  final  molecule  
(10) Nucleic   acids   –   nucleotides   joined   together   by   the   Pi   joining   with   3rd   carbon   of  
pentose  sugar  by  a  condensation  reaction  to  form  a  phosphodiester  bridge  (resulting  
in  a  repeating  sugar-­‐phosphate  chain)  between  the  third  and  fifth  carbons  

(11) Structure  of  DNA  –  polymer  of  2  chains  of  nucleic  acids  running  antiparallel  to  one  
another  and  twisted  into  a  double  helix  

(a) The  two  strands  are  linked  by  paired  bases  which  project  towards  each  other  and  
are  held  together  by  hydrogen  bonds  (making  DNA  a  very  stable  structure)  
(b) Pyrimidine  always  bonds  with  a  purine  –  keeping  constant  width  between  strands  
(c) Nucleotides   with   adenine   can   make   two   hydrogen   bonds   with   nucleotides   with  
thymine  (or  uracil  in  the  case  of  RNA)  as  the  base  
(d) Nucleotides  with  guanine  can  make  three  hydrogen  bonds  with  nucleotides  with  
cytosine  as  the  base  
(e) Chains  run  antiparallel  so  one  end  finishes  on  the  3rd  carbon  and  the  other  on  the  
5th  –  the  strands  run  and  the  sugars  point  in  opposite  directions  to  each  other  
(f) The  antiparallel  chains  twist  like  a  rope  ladder  to  form  a  final,  double  helix  
 
iv) Describe,   with   the   aid   of   diagrams,   how   the   sequence   of   nucleotides   within   a   gene   is  
used   to   construct   a   polypeptide,   including   the   roles   of   messenger   RNA,   transfer   RNA   and  
ribosomes  
(1) Messenger   RNA   –   single   strand   of   nucleotides   where   uracil   replaces   thymine   and  
ribose  sugar  replaces  deoxyribose  sugar  (copy  of  the  DNA  coding  strand),  responsible  
for  bringing  the  information  contained  in  DNA  out  of  the  nucleus  to  the  ribosome    

 
(2) Transfer  RNA  –  carries  the  anti  codon  and  the  specific  amino  acids  to  the  ribosomes  
where  they  are  bonded  together  to  form  polypeptides  

(a) Lengths  of  RNA  fold  into  hairpin  shapes  
(b) Have  three  exposed  bases  at  one  end  where  a  particular  amino  acid  can  bind  
(c) At  the  other  end,  there  are  three  unpaired  nucleotide  bases  –  anticodon  
(3) Ribosomal   RNA   –   spherical   with   a   small   sub-­‐unit   on   left   and   large   sub-­‐unit   on   the  
right   (found   in   ribosomes),   forming   part   of   the   structure   of   a   ribosome   and  
accounting  for  80%  of  the  total  RNA  in  a  cell  
(4) Protein  synthesis  
(a) DNA  à  mRNA  à  tRNA  à  polypeptide  
(b) Transcription   –   creation   of   a   single-­‐stranded   mRNA   copy   of   the   DNA   coding  
strand  (which  is  small  enough  to  leave  the  nucleus)  
(i) Genetic   code   in   nucleus   of   cell   contains   the   information   for   making  
polypeptides  
(ii) The   sequences   of   bases   on   DNA   make   up   codes   for   particular   protein  
molecules,   acting   as   a   template   strand   –   coding   for   the   sequence   of   amino  
acids  in  the  protein  
(iii) The   gene   to   be   transcribed   unwinds   and   unzips   by   being   dipped   into   the  
nucleus  so  hydrogen  bonds  between  complementary  bases  break  
(iv) The  sequence  for  a  particular  gene  is  exposed  
(v) Free   floating   DNA   nucleotides   are   found   in   the   nucleoplasm   and   free   RNA  
nucleotides  are  found  in  the  nucleolus  
(vi) The  nucleotides  are  activated  to  ATP,  GTP,  CTP  and  UTP  –  they  have  two  extra  
phosphoryl  groups  attached  
(vii) Free  activated  RNA  nucleotides  pair  up  and  bind  temporarily  with  hydrogen  
bonds   to   their   exposed   complementary   bases   (G   to   C   and   A   to   U)   on   the  
template  strand  of  unwound  DNA  
(viii) The  two  extra  phosphoryl  groups  are  released,  releasing  energy  for  bonding  
adjacent  nucleotides  
(ix) Their   sugar   phosphate   groups   are   bounded   together   to   form   the   sugar-­‐
phosphate  backbone  via  condensation  reactions  
(x) A  single-­‐stranded  piece  of  mRNA  thus  forms,  catalysed  by  RNA  polymerase    
(xi) The  mRNA  is  released  from  the  DNA  and  leaves  the  nucleus,  through  a  pore  in  
the  nuclear  envelope,  to  a  ribosome  
(c) Translation  –  the  formation  of  a  sequence  of  amino  acids  to  form  a  polypeptide  
on  a  ribosome  using  mRNA  as  the  code,  whereby  the  amino  acids  are  assembled  
into  the  sequence  dictated  by  the  sequence  of  codons  on  the  mRNA  
(i) NB:  in  prokaryotes,  translation  starts  as  soon  as  some  mRNA  has  been  made  
(ii) Ribosomes  are  assembled  in  the  nucleolus  of  eukaryote  cells  from  ribosomal  
RNA  and  protein  –  2  sub-­‐units  and  groove  into  which  a  length  of  mRNA  can  fit  
(iii) The  ribosome  has  two  sites  for  amino  acids  to  bind  on  the  ribosomal  groove  
(iv) mRNA   peels   away   from   the   DNA   and   passes   from   the   nucleus   to   the  
cytoplasm  by  a  nuclear  pore,  carrying  the  code  to  ribosomes  in  the  cytoplasm  
(v) Ribosome  assembles  around  the  mRNA  and  moves  along  three  bases  at  a  time  

(vi) tRNA  is  made  in  the  nucleus  and  passes  into  the  cytoplasm    
(vii) The  end  of  the  tRNA  molecule  has  an  anticodon  (three  exposed  bases)  –  61  
different  tRNA  molecules  exist  (a  codon  –  a  triplet  of  three  bases  on  mRNA)  
(viii) Activation  –  each  tRNA  molecule  binds  to  a  specific  amino  acid  and  brings  it  
to  the  ribosome  according  to  the  base  sequence  on  the  mRNA  
1
...
The  tRNA  molecule  is  then  said  to  be  charged  
(ix) The  first  exposed  mRNA  codon  is  always  AUG  
(x) Using   ATP   and   an   enzyme,   a   tRNA   with   methionine   and   the   complimentary  
anticodon   UAC   forms   hydrogen   bonds   with   this   codon   in   the   ribosome’s  
binding  site  
(xi) The  second  site  is  now  filled  by  the  appropriate  tRNA,  bringing  the  next  amino  
acid  to  the  ribosome  as  specified  by  the  mRNA  codon  
(xii) The   adjacent   amino   acids   attached   to   the   tRNA   molecules   are   joined  
together   by   a   peptide   bond,   catalysed   by   an   enzyme   in   the   small   ribosomal  
sub-­‐unit  
(xiii) The   first   tRNA   molecule   is   now   ‘empty’   and   leaves   the   ribosome   to   return   to  
the  cytoplasm  to  pick  up  a  further  amino  acid  
(xiv) The  ribosome  then  moves  along  the  mRNA  by  a  distance  of  one  codon  
(xv) This   leaves   the   second   site   for   amino   acid   binding   empty   and   so   a   third  
amino  acid  can  be  added  to  the  growing  polypeptide  chain  
(xvi) Elongation  of  the  polypeptide  chain  continues  until  a  STOP  codon  is  reached  
(UAA,  UAG,  UGA)  –  no  corresponding  tRNAs  for  these  three  codons  
(xvii) Polypeptide  chain  separates  from  ribosome  –  protein  synthesis  is  complete  
(xviii) Sequence  of  amino  acids  in  a  protein  is  critical  because…  
1
...
The   specific   primary   structure   gives   rise   to   the   secondary   and   tertiary  
structure  of  the  protein  appropriate  to  its  function  
3
...
The  tertiary  structure  of  a  protein  is  what  allows  a  protein  to  function  
5
...
 changing  the  shape  of  the  active  site  of  an  enzyme  
 
v) State  that  mutations  cause  changes  to  the  sequence  of  nucleotides  in  DNA  molecules  
(1) Mutation  –  any  random  change  in  the  structure  or  amount  of  DNA  in  an  organism  ie
...
 tobacco,  UV  light,  X-­‐rays,  gamma  rays  
(2) Gene  Mutation  –  a  change  in  the  structure  of  DNA  ie
...
  substitutions   –   a   single   base   pair   is   replaced   by   another,  
substituted  for  another  base  
(i) Missense  mutation  –  arises  if  another  amino  acid  is  coded  for  as  a  result  
1
...
 
UAA,  UAG,  UGA  
1
...
This  can  yield  a  truncated/abbreviated  protein  product  
3
...
Eg
...
 a  base  is  removed  
(iv) Results   in   a   frame   shift   –   might   appear   to   make   sense,   but   in   fact   it   can   result  
in  the  production  of  an  entirely  different  polypeptide  
(v) Eg
...
 Sickle  cell  anaemia  –  point  mutation  
(i) On  codon  6  of  the  gene  for  the  beta  polypeptide  chains  of  haemoglobin  
(ii) The  replacement  of  A  by  T  in  the  17th  nucleotide  (ie
...
 Cystic  Fibrosis  –  deletion    
(i) 70%  of  cases  is  due  to  the  mutation  of  a  triplet  of  base  pairs    
(ii) 1   amino   acid   from   the   sequence   of   1480   amino   acids   in   the   normal  
polypeptide  chain  is  deleted  

(f) Eg
...
 Huntington  disease  –  stutter  
(i) Results  from  an  expanded  triple  nucleotide  repeat  
(ii) Normal  gene  has  repeating  CAG  sequences  
(iii) Protein  is  altered  sufficiently  when  CAG  sequences  are  expanded  to  above  a  
threshold  number  
(iv) Symptoms  manifest  later  in  life  eg
...
 In  the  number  or  structure  of  chromosomes    
(b) Includes  a  deletion,  inversion  or  translocation    
(c) The   most   common   chromosome   mutation   is   trisomy   21   aka
...
 although  the  base  triplet  has  changed,  it  still  codes  
for  the  same  or  similar  amino  acid  so  the  protein  is  unchanged  
(c) The  mutation  may  cause  a  change  to  the  structure  of  the  protein,  and  therefore  a  
different   characteristic,   but   the   changed   characteristic   gives   no   particular  
advantage  or  disadvantage  to  the  organism    
(i) Eg
...
 Ability  to  roll  your  tongue  (genetic  trait)  
(iii) Eg
...
 Melanin  in  skin  
(b) Early   humans   in   Africa   had   dark   skin   –   pigment   melanin   protected   them   from  
harmful  effects  of  UV  light  whilst  still  being  able  to  synthesise  vitamin  D  
(c) Any   humans   with   mutations   to   some   of   the   genes   determining   skin   colour,  
producing  paler  skin,  would  have  burned  and  suffered  from  skin  cancer  
(d) As   humans   migrated   to   more   temperate   climes,   the   sunlight   was   not   intense  
enough  to  cause  enough  vitamin  D  to  be  made  by  those  with  dark  skin  
(e) Humans  with  a  lack  of  melanin  could  synthesise  more  vitamin  D  

(f) Those  without  a  lack  of  melanin  and  a  deficiency  in  vitamin  D  would  suffer  from  
rickets  and,  in  females,  a  narrow  pelvis  producing  difficulties  in  childbirth  
(3) Depending  on  the  environment,  the  same  mutation  for  paler  skin  can  be  beneficial  or  
harmful  –  the  environment  is  never  static  
(4) Natural  selection  –  the  mechanism  for  evolution  
(a) Individuals   within   a   population   who   have   a   certain   characteristic   may   be   better  
adapted  to  the  new  environment  
(b) The  well-­‐adapted  organisms  with  the  advantageous  allele  can  out-­‐compete  those  
in  the  population  that  do  not  have  the  advantageous  characteristic  
(c) Thus,  without  genetic  mutations  there  would  be  no  evolution  
 
vii) State  that  cyclic  AMP  activates  proteins  by  altering  their  three-­‐dimensional  structure  
(1) Cyclic   AMP   (from   ATP)   –   some   proteins   have   to   be   activated   by   chemicals,   which  
change  their  3D  shapes  so  they  are  a  better  fit  to  their  complementary  molecules  
(2) The   presence   or   absence   of   glucose   affects   the   concentration   of   cyclic   AMP   in   the  
cell,  and  therefore  the  lac  operon  
(3) Concentration  of  cyclic  AMP  in  E
...
 code  for  proteins  eg
...
 Coli  bacteria  cells  –  converted  into  glucose  and  galactose  
(h) Sugars  can  then  be  used  for  respiration  to  produce  energy  from  the  lactose  
(i) As  the  concentration  of  lactose  decreases,  there  are  less  repressor  proteins  with  
bound  lactose  
(j) Transcription  of  structural  genes  therefore  eventually  stops  
(7) In  absence  of  lactose,  with  the  agar  containing  glucose  as  the  only  energy  source  
(a) The  regulator  gene  is  expressed  (transcribed  and  translated)  
(b) The   repressor   protein   is   synthesised   with   two   binding   sites   (one   that   binds   to  
lactose  and  one  that  binds  to  the  operator  region)  
(c) The  repressor  protein  binds  to  the  operator  region  
(d) Part  of  promoter  region  is  covered  –  where  RNA  polymerase  normally  attaches  
(e) This  prevents  RNA  polymerase  from  binding  to  the  promoter  region  
(f) Structural  genes  cannot  be  transcribed  into  mRNA  
(g) Translation  of  proteins/enzymes  required  to  break  down  lactose  is  blocked  
 

ix) Explain   that   the   genes   that   control   development   of   body   plans   are   similar   in   plants,  
animals  and  fungi,  with  reference  to  homeobox  sequences  (HSW1)  
(1) Process  of  cell  differentiation  
(a) Much  of  what  is  known  about  the  molecular  basis  of  cell  differentiation  has  come  
from  the  study  of  the  fruit  fly  –  Drosophila  melanogaster  
(b) DNA   replication   in   the   Drosophila   eggs   is   faster   than   is   known   for   any   other  
organism  (one  every  6-­‐10  minutes)  
(c) A   series   of   mitotic   divisions,   which   are   not   followed   by   cell   division,   begin   after  
the  eggs  are  laid  
(d) A  syncytium  is  formed  –  a  multinucleate  structure  
(e) After  the  8th  division,  the  256  nuclei  migrate  from  the  middle  of  the  cells  to  the  
outer  layer  
(f) By  the  11th  division,  the  nuclei  form  a  single  layer  around  a  yolk  filled  core  
(g) The  division  rate  slows  (the  14th  division  takes  60  minutes)  
(h) The  nuclear  genes  switch  from  replicating  to  transcribing  
(i) Plasma   membranes   invaginates   (folds   inwards)   from   the   cell   surface   and  
surround  each  individual  nucleus  to  form  about  6000  cells  
(j) About  15  nuclei,  found  at  the  extreme  posterior  of  the  cell,  form  the  pole  cells  –  
primordial  germ  cells  that  will  give  rise  to  the  eggs  or  sperm  
(k) After   another   2-­‐3   hours,   the   embryo   divides   to   form   a   series   of   segments   –  
corresponding  to  the  organism’s  organisation/body  plan  
(l) The  three  Md,  Mx  and  Lb  segments  merge  to  produce  the  head  
(m) T1-­‐3  are  the  thorax  segments  
(n) A1-­‐8  are  the  abdominal  segments  
(o) Metamorphosis  –  larval  form  becomes  the  adult  as  the  legs,  wings  and  antenna  
develop  

(2) Three  main  stages  of  development  
(a) Development  of  axes  –  anterior,  posterior,  dorsal  and  ventral  
(b) Segmentation  –  division  of  body  into  head,  abdomen  and  thorax  
(c) Segment  identity  –  structures  specific  for  each  segment  developed  
(3) Controlling  cell  development  and  an  organism’s  body  plan  using  homeobox  genes  
(a) Homeobox  genes  –  code  for  polypeptides  that  act  as  transcription  factors,  turning  
transcription  of  other  genes  on  or  off  in  the  order  that  they  are  expressed  
(i) Found  in  all  segmented  animals,  plants  and  fungi  
(ii) Contain  a  sequence  of  180  base  pairs,  therefore  coding  for  60-­‐amino  acid  long  
polypeptide  chains  
(iii) Arranged   in   Hox   clusters   –   as   the   complexity   of   the   organism   increases,   the  
number  of  Hox  clusters  the  organism  has  also  increases  
(b) Maternal  effect  genes  –  controls  development  of  axes/the  embryo’s  polarity  
(i) Transcribes  mRNA  
of  genes  that  allow  
axes  to  be  
established  
(ii) Come  from  the  
mother  who  puts  
mRNA  into  each  egg  
before  fertilisation  
(iii) mRNA  codes  for  
proteins  eg
...
 Kruppel  gene  –  responsible  for  development  of  embryo’s  middle  region  
(c) Homeotic   selector   genes   –   specify   the   identity   of   each,   individual   segment   and  
direct  their  development  including  the  formation  of  appendages    
(i) The  master  genes  in  the  control  networks  of  regulatory  genes  
(ii) Some  regulate  the  development  of  the  thorax  and  abdomen  segments  
(iii) Others  regulate  the  development  of  the  head  and  thorax  segments  
(d) Antennapedia  –  mutation  in  the  homeotic  genes  can  change  one  body  part  into  
another  eg
...
 CNS  
(b) Retinoic   acid   is   a   morphogen   –   substance   that   governs   the   pattern   of   tissue  
development  ie
...
 cranial  deformities  
 
x) Outline  how  apoptosis  (programmed  cell  death)  can  act  as  a  mechanism  to  change  body  
plans  
 
(1) Apoptosis  –  a  series  of  biochemical  events  that  together  are  a  fast  process  of  
programmed  cell  death  that  occurs  in  multicellular  organisms  
(2) Hayflick  constant  –  cells  should  undergo  about  50  mitotic  divisions  and  then  undergo  
a  series  of  biochemical  events  that  leads  to  an  orderly  and  tidy  cell  death  
(a) Cancer  cells,  on  the  other  hand,  are  immortal  
(b) The  rate  of  cells  dying  should  
balance  the  rate  of  cells  
produced  by  mitosis  
(c) Not  enough  apoptosis  leads  to  
the  formation  of  tumours  
(d) Too  much  apoptosis  leads  to  
cell  loss  and  degeneration  
(3) Necrosis  –  an  untidy  and  damaging  
cell  death  that  occurs  after  trauma  
and  releases  hydrolytic  enzymes  
(4) Mechanism  for  apoptosis  
(a) Enzymes  break  down  the  cell  
cytoskeleton  
(b) Cell  surface  membrane  changes  
(c) Cytoplasm  becomes  dense,  with  
organelles  tightly  packed  
(d) Chromatin  condenses    
(i) Nuclear  envelope  breaks  
(ii) DNA  breaks  into  fragments  
(e) Blebs  –  cell  breaks  up  into  small  
vesicles  
(f) Vesicles  are  taken  up  by  
phagocytosis  
(5) Controlling  apoptosis  
(a) Events  are  controlled  by  a  
diverse  range  of  internal  and  
external  cell  signals  

(b) Eg
...
  (Names   of   the   main   stages   are   expected,   but   not   the   subdivisions   of  
prophase)  
(1) Binary  Fission  –  reproduction  in  prokaryotes  producing  genetically  identical  offspring  
(a) Single  DNA  molecule  replicates  
(b) Both  copies  attach  to  the  cell  membrane  
(c) Cell  membrane  begins  to  grow  between  the  two  DNA  molecules  
(d) Bacterium  doubles  in  size  
(e) Cell  membrane  pinches  inward  
(f) Cell   wall   forms   between   two   DNA   molecules   dividing   the   original   cell   into   two  
identical  cells  
(g) Genetic  variation  will  only  be  introduced  by  mutation  
(2) Mitosis  –  asexual  reproduction  in  eukaryotes  
(a) Produces  two  genetically  identical  cells  
(b) Each  daughter  cell  contains  the  same  number  and  types  of  chromosomes  as  the  
original  parent  cell  
(3) Meiosis  is  a  reduction  division  –  sexual  reproduction  in  eukaryotes  
(a) Occurs  in  the  sex  organs  ie
...
 centrioles  and  mitochondria,  replicate  
(v) ATP  is  produced  
(b) Meiosis  I  –  homologous  chromosomes  pair  up  and  swap  portions  of  chromatids  
(crossing  over)  before  separating  (the  cell  splits  in  two)  
(i) Prophase  I  
1
...
Chromosomes   condense   (shorten   and   thicken)   by   supercoiling   and  
become  visible  –  can  take  up  stains  and  be  seen  with  a  light  microscope  
3
...
Non-­‐sister   chromatids   wrap   around   each   other   and   attach   at   crossover  
points  (chiasmata)  
5
...
Centrioles   migrate   to   opposite   poles   of   the   cell   and   start   to   produce  
spindle  fibres  made  of  protein  microtubules  
(ii) Metaphase  I  
1
...
Members  of  the  homologous  pairs  face  opposite  poles  –  so  chromosomes  
can  independently  segregate  when  pulled  apart  in  anaphase  I  
3
...
Chiasmata  are  still  present  
(iii) Anaphase  I  
1
...
Chromosomes  move  to  opposite  poles  of  the  cell  
3
...
Chiasmata  separate  
5
...
Plant  cells  skip  telophase  I  and  go  from  anaphase  I  to  meiosis  II    
2
...
Chromosomes  arrive  at  opposite  poles  of  the  cell  
4
...
May  or  may  not  be  followed  by  cell  division  (cytokinesis)  
6
...
If  a  nuclear  membrane  has  reformed,  it  breaks  down  again  
2
...
Chromosomes  relax  and  then  recondense  
4
...
Chromosomes  line  up  along  the  equator  of  the  spindle  
2
...
Chromosomes  are  attached  to  the  spindle  fibres  at  the  centromeres  
(iv) Anaphase  II  
1
...
Centromeres  splits  
3
...
Once  pulled  apart,  chromatids  are  known  as  chromosomes  
(v) Telophase  II  
1
...
A  new  nuclear  membrane  reforms  around  each  set  of  chromosomes  
3
...
In  plants,  a  tetrad  (group  of  four)  haploid  cells  is  formed  
 

ii) Explain   the   terms   allele,   locus,   phenotype,   genotype,   dominant,   codominant   and  
recessive  
(1) Gene  –  length  of  DNA  that  codes  for  one  or  more  polypeptides  
(2) Allele  –  alternative  versions  of  a  particular  gene  eg
...
  red   allele   and  
white  allele  will  be  both  expressed  in  flowers  and  produce  a  pink  flower  
(a) In   horses   –   chestnut   and   white   colours   are   codominant   so   heterozygous   horses  
have  a  blend  of  both  colours  and  are  golden,  tan  palominos  
(10) Recessive   –   allele   only   expressed   in   the   phenotype   when   it   is   present   in   the  
homozygous   state   (with   another   identical   allele)/in   the   absence   of   the   dominant  
allele  for  the  same  gene  
(11) Autosomes  –  chromosomes  not  concerned  with  determining  sex  
 
iii) Explain  the  terms  linkage  and  crossing-­‐  over  
(1) Linkage  –  two  or  more  genes  that  are  located  on  the  same  chromosome  
(a) Reduces  the  number  of  phenotypes  resulting  from  a  cross  
(b) Linked   alleles   with   no   chiasmata   formed   between   them   are   inherited   together  
because  they  do  not  segregate  independently  during  meiosis  
(c) The  closer  together  genes  are  on  a  chromosome,  the  less  chance  there  is  of  them  
being  separated  by  crossing  over  
(d) Crossing  over  would  otherwise  produce  recombinants  
(2) Sex  linkage  –  characteristic  which  is  coded  for/determined  by  a  gene  that  is  found  on  
the  sex  (either  the  X  or  Y)  chromosomes  
(a) In  most  animals,  the  small  Y  chromosome  has  few  genes  
(b) Most  sex-­‐linked  genes  are  likely  to  be  found  on  the  X  chromosome  
(c) Eg
...
 human  blood  groups  IA,  IB  and  IO  
(e) In  the  case  of  codominance,  the  genes  is  also  given  an  upper  case  letter  and  the  
alleles  are  superscripted  
(3) Pedigree  diagrams  
(a) Pedigree  analysis  –  one  of  the  most  powerful  tools  in  human  genetic  studies  

(4) Characteristics  of  genetic  diseases  controlled  by  a  single  recessive  gene  
(a) Children  born  to  two  carriers  have  a  25%  chance  of  being  homozygous  recessive  
therefore  have  the  disorder  
(b) Carriers  will  not  have  the  disorder  
(c) Eg
...
Mutation   disrupts   the   transport   of   chloride   ions   and   water   across   the  
membranes  of  cells  lining  the  airways,  gut  and  reproductive  tracts  
2
...
Cilia  are  not  properly  hydrated  and  cannot  shift  mucus  
(ii) Effect/symptoms  
1
...
More  likely  to  get  respiratory,  bacterial  infections  
3
...
Affected  gut  and  pancreas  so  food  is  not  digested  efficiently  
5
...
Homozygous  dominant  –  all  chloride  ion  channels  function  
2
...
Heterozygous   –   no   symptoms   but   do   have   some   abnormal   chloride   ion  
channels  yet  enough  normal  channels  for  their  lungs  to  functions  
(d) Eg2
...
Factor   VIII   –   blood   clotting   protein   coded   for   by   a   gene   on   the   X  
chromosome  
2
...
Hemizygous  –  males  have  only  one  X  chromosome,  if  it  has  the  allele  for  
haemophilia  A,  the  male  will  suffer  from  haemophilia  
(ii) Effect  
1
...
Internal   bleeding   from   knocks   and   bleeding   into   joints   are   particularly  
harmful  
(e) Eg3
...
Dystrophin  –  DMD  gene  for  a  muscle  protein  on  the  X  human  chromosome  
2
...
Mutations   of   the   gene   usually   result   in   a   severely   truncated   dystrophin  
protein  or  no  dystrophin  at  all  (deletion  mutation)  
(ii) Effect  
1
...
Usually  wheelchair-­‐bound  by  10  years  old  

3
...
 Hypercholesterolemia  –  heterozygote  has  a  reduced  number  of  functional  low  
density  lipoprotein  receptors  
(b) Individuals  possessing  one  copy  of  the  affected  allele  will  have  the  disease  
(c) One   carrier   parent   and   one   normal   parent   have   a   50%   change   of   producing   an  
affected  child  
(d) If  even  one  affected  parent  is  homozygous,  all  offspring  will  be  affected  
(6) Characteristics  of  traits  carried  on  the  X  chromosome  –  sex  linked  eg
...
 Sickle  cell  anaemia  
(i) Cause  
1
...
On  codon  6  of  the  gene  for  the  beta  polypeptide  chains  of  haemoglobin  
3
...
  6th   amino   acid)  
changes  GAG  to  GTG  
4
...
Deoxygenated,   abnormal   haemoglobin   is   not   soluble   and   becomes  
crystalline  –  aggregating  into  more  linear  and  less  globular  structures  
2
...
Sticky,  sickle  cells  are  not  adapted  to  carrying  oxygen  or  squeezing  through  
the  capillaries  
4
...
Eventually  organs  (especially  heart,  lungs  and  kidneys)  become  damaged  
6
...
HAHA  –  normal  haemoglobin  
2
...
HAHS  –  half  the  haemoglobin  is  normal  and  half  is  sickled  when  red  blood  
cells  are  made  in  the  bone  marrow  
4
...
At  whole  organism  level  this  condition  could  be  considered  to  be  recessive  
6
...
 Shorthorn  cattle  and  one  of  the  genes  for  coat  colour  
(i) When  complete  dominance  is  lacking,  upper  and  lower  case  are  not  used  to  
represent  the  genes  
(ii) Homozygous  individuals  with  genotype  CRCR  have  red  (chestnut)  coats  
(iii) Homozygous  individuals  with  genotype  CWCW  have  white  coats  
(iv) Heterozygous   individuals   with   genotype   CRCW   have   red   and   white   hairs   and  
the  coat  is  roan  –  roan  cattle  
(c) Eg3
...
 The  ABO  blood  system  classifies  red  blood  cells  according  to  antigens  on  the  
cell  surface  membranes  
(f) The  antigens  are  determined  by  gene  ‘I’  which  has  three  alleles  (IO,  IA,  IB)  
(g) IA  and  IB  are  codominant  but  are  both  dominant  to  IO  
(9) Linkage   –   when   both   genes   are   carried   on   the   same   chromosome   so   they   are  
segregated  together  
(a) Monohybrid  –  gene  that  codes  for  one  characteristic  
(b) Dihybrid  –  two  genes  controlling  two  different  characteristics  
(i) Eg
...
 Completely  random,  independent  assortment  
(c) Linkage  group  –  group  of  genes  that  are  normally  inherited  together  because  they  
are  found  on  the  same  chromosome  
(i) Alleles  are  linked  on  the  same  chromosome  
(ii) Not  independently  assorted  during  meiosis  I  
(iii) Cross  two  heterozygotes  for  two  genes  
(iv) Only  two  gametes  are  produced  –  YG  and  yg  (not  YG,  Yg,  yG,  yg)  

(v) Recombinants   can   occur   in   low   frequencies   due   to   crossing   over   –   can  
separate  two  linked  genes  
1
...
Genes  that  are  close  together  rarely  get  separated  by  crossing  over  
(vi) Not  the  expected  9:3:3:1  dihybrid  and  unlinked  ratio    
 
vi) Describe   the   interactions   between   loci   (epistasis)
...
 Salvia  
1
...
aa  –  epistatic  to  both  alleles  of  the  gene  B/b  
3
...
 Leghorn  and  White  Wyandotte  chickens  
1
...
C  –  coloured  feathers  
3
...
 Sweet  peas  –  purple  colour  is  produced  by  the  action  of  two  enzymes  (C  +  P)  
(c) If  either  gene  is  homozygous  recessive,  the  purple  colour  will  not  be  produced  
 
viii) Use   the   chi-­‐squared   (χ2)   test   to   test   the   significance   of   the   difference   between   observed  
and  expected  results
...
 height,  weight,  arm  length  
(a) Describes  quantitative  differences  in  phenotypes  
(b) No  distinguishable  categories  
(c) Wide  variation  in  the  population  
(d) Frequency  distribution  =  normal  distribution  
(e) Mode  (most  frequently  occurring  value)  falls  in  the  middle  of  the  range  (median)  
which  is  the  same  as  the  mean  
(f) Most   of   the   organisms   in   the   population   fall   in   the   middle   of   the   range   with  
approximately  equal  numbers  showing  the  two  extremes  
(4) Discontinuous  variation  –  limited  number  of  distinct  forms  within  a  population  with  
no  intermediates  in  between  eg
...
 Only  one  gene  is  involved  in  the  inheritance  of  the  CFTR  gene,  inheritance  of  
two  faulty  genes  results  in  cystic  fibrosis  
(d) If  there  is  more  than  one  gene  involved  –  genes  interact  in  an  epistatic  way  
(e) Different  alleles  at  a  single  gene  locus  have  large  effects  on  the  phenotype  
(f) Different  gene  loci  have  quite  different  effects  on  the  phenotype  
 
xi) Explain   that   both   genotype   and   environment   contribute   to   phenotypic   variation
...
 tall  genes  won’t  
be  fully  expressed  if  you  are  undernourished  at  crucial  stages  in  your  development  
 
xii) Explain  why  variation  is  essential  in  selection  
(1) Variation   –   raw   material   for   both   natural   and   artificial   selection   (a   plausible  
mechanism  for  evolution)  
(a) Individuals  within  a  species  differ  from  each  other  –  variation    
(b) Offspring  resemble  their  parents  –  inherit    
(c) Far   more   offspring   are   generally   produced   than   survive   to   maturity   –   they   may  
suffer  from  predation,  disease  or  competition  
(d) Populations  are  usually  fairly  constant  in  size  
(2) Process  of  natural  selection  
(a) The  environment  changes  
(b) Selection   pressure   –   an   environmental   factor   that   confers   greater   chances   of  
surviving  to  reproductive  age  on  some  members  of  the  population  
(i) Eg
...
If  mutation  does  occur  –  forward  and  backward  mutations  are  equal  
(vi) No   genetic   drift   –   no   changes   in   allele   frequencies   resulting   from   random,  
chance  processes  (more  likely  to  affect  smaller  populations)  
(vii) Characteristic  being  studied  is  not  sex-­‐linked  –  only  for  traits  with  dominant  
and  recessive  alleles  
 
xiv) Explain,  with  examples,  how  environmental  factors  can  act  as  stabilising  or  evolutionary  
forces  of  natural  selection  
(1) Population  growth  
(a) All  organisms  can  reproduce  –  have  the  potential  to  increase  their  population  size  

(b) Over  time,  the  population  size  will  fluctuate  around  a  mean  level  
(c) Great  environmental  resistance  –  population  size  will  shrink,  competition  will  be  
reduce  and  the  population  will  grow  
(d) As   the   population   increases   –   more   intraspecific   competition   for   resources   and  
the  population  size  will  fall  again  
(e) Carrying   capacity   –   maximum   size   that   the   environment   can   sustain   and   then  
remain  stable  
(f) In   this   case,   not   all   the   young   produced   survive   to   adulthood   –   otherwise   they  
would  produce  young  too  and  the  population  would  continue  to  expand  
(g) Natural  selection  can  affect  allele  frequencies  in  several  different  ways  
(h) For   continuously   varying   characteristics,   these   selection   methods   modify   their  
frequency  distributions  in  different  ways  
(2) Stabilising  selection  –  natural  selection  keeps  things  the  way  they  are  
(a) Acts  against  the  extremes  within  a  range  of  phenotypic  variation  
(b) Leads  to  a  reduction  in  the  range  of  variation  within  the  population  without  any  
change  in  the  mode  
(c) Operates  in  an  unchanging  environment  to  maintain  the  best  adapted  genotypes  
(d) Eg
...
  Agouti   coat   can   camouflage   rabbits   and   prevent   predation,   white   coat  
rabbits  will  be  advantageous  if  the  environment  changes  and  snow  then  falls  
(f) Environmental  factors  –  caused  by  abiotic  or  biotic  components  
(i) Space   –   for   plants   to   grow,   for   animals   to   defend   a   feeding   territory,   for  
animals  to  rear  young  
(ii) Availability  of  food,  light,  minerals  or  water  
(iii) Predation  
(iv) Infection  by  pathogens  

(3) Directional   selection   –   evolutionary   force   of   natural   selection   that   leads   to   an  
evolutionary  change  
(a) Operates  in  changing  environments  
(b) Acts  for  or  against  extremes  of  phenotypes  within  the  variable  population  

(c) Main   type   of   selection   practised   by   man   when   selecting   domesticated   plants   and  
animals  for  the  improvement  of  stocks  
(d) Reduction  in  the  range  of  variation  within  a  population  
(e) Progressive  shift  in  the  mode  

(f) Eg
...
 Copper  tolerance  in  grasses  
(i) Copper   tolerance   is   a   genetically-­‐determined   quantitative   character,  
displaying  many  degrees  of  tolerance  (polygenic  inheritance)  
(ii) Soil  in  and   around  old  copper  mines  contains  high  concentrations  of  copper  –  
normally  toxic  to  most  species  of  plants  and  animals  
(iii) Agrostis  tenuis  –  copper-­‐tolerant  plant  found  growing  in  areas  of  land  where  
the  soil  is  contaminated  with  copper  
(iv) Two  varieties  of  Agrostis  tenuis  are  found  growing  around  old  copper  mines  
(v) Non-­‐tolerant   plants   are   unable   to   grow   in   the   copper-­‐rich   soil,   but   tolerant  
plants  are  able  to  grow  in  normal  soil  
1
...
Samples   of   mature   plants   taken   from   the   copper-­‐rich   soil   of   the  
abandoned   mine   show   a   high   level   of   tolerance   to   copper   and   a   small  
range  of  variation  

(vi) When   seeds   were   collected   from   the   sampled   copper-­‐tolerant   plants   and  
grown   in   ordinary   soil,   variation   in   tolerance   is   much   greater   –   no   selection  
pressure/forces  for  copper  tolerance  
1
...
 due  to  a  
natural  disaster  or  disease  pandemic  
(ii) Eg
...
016  
(vi) As  5%  currently  suffer  from  the  disorder,  current  allele  frequency  =  0
...
 as  they  may  not  recognise  each  other’s  courtship  behaviour  
(i) Eg
...
When   the   finches   reproduced,   some   of   the   alleles   would   be   lost   as   a  
natural  part  of  how  gametes  randomly  fuse  –  down  to  chance  
2
...
Not  all  the  offspring  will  survive  –  also  reducing  the  number  of  alleles  
4
...
 Rising  sea  level  isolates  a  population  on  an  island  into  three  sub-­‐populations  
(i) The   separated   islands   become   different   environments   as   climatic   and   other  
factors  influence  the  physical  nature  of  the  environment  
(ii) Natural  selection  acts  in  different  ways  on  the  variable  population  
(iii)  Differences   in   their   genetic   composition   are   promoted   so   they   can   adapt  
independently  to  each  environment  
(iv) Eventually   the   sub-­‐populations   diverge   to   such   an   extent   that   they   become  
reproductively  isolated  from  one  another  and  become  different  species  
(d) Egs
...
 incompatible  genitals  
 
xvii) Explain   the   significance   of   the   various   concepts   of   the   species,   with   reference   to   the  
biological   species   concept   and   the   phylogenetic   (cladistic/evolutionary)   species   concept  
(HSW1)  
(1) Species   can   be   regarded   as   the   starting   point   in   classification   of   living   organisms   –  
group   of   similar   organisms   that   can   interbreed   and   produce   fertile   offspring  
(reproductively  isolated  from  other  such  groups)  
(2) Biological  species  concept  –  a  group  of  similar  organisms  that  interbreed  to  produce  
fertile  offspring  and  are  reproductively  isolated  from  other  such  groups  
(a) Problematic  when  deciding  which  species  an  organism  belongs  to/new  species  

(b) Biologists  want  to  classify  living  organisms  that  do  not  reproduce  sexually  –  never  
reproduce  together  even  if  they  belong  to  the  same  species  eg
...
 males  
and  female  ants  have  very  different  castes  
(d) Might  be  extinct  –  can’t  study  their  reproductive  behaviour  
(e) Practical/ethical   issues   –   geography   not   allowing   you   to   see   if   organisms  
reproduce  successfully  in  the  wild,  can’t  study  them  in  a  lab  
(3) Phylogenetic/cladistic/evolutionary   species   concept   –   hierarchical   classification   of  
species   based   on   the   study   of   the   evolutionary   ancestry   of   groups   of   organisms  
(what  is  related  to  what  and  how  closely)  
(a) Basics  
(i) Share   similar   morphology   (shape),   physiology   (biochemistry),   embryology  
(stages  of  development)  and  behaviour,  and  occupy  the  same  ecological  niche  
(ii) All  evolved  from  shared,  common  ancestors  
(iii) The  more  closely  related  two  species  are,  the  more  recent  their  last  common  
ancestor  will  be  
(iv) Which   species   or   new   species?   –   close   relations   to   members   of   another  
species,  then  its  probably  the  same  species  
(v) Difficulty  –  no  cut  off  point  to  say  how  different  two  organisms  have  to  be  to  
be   different   species   eg
...
Differences  are  caused  by  base  substitutions  –  expressed  as  %  divergence  
2
...
 of  substitutions  /  no
...
 Grouping  of  reptiles  excludes  birds  which  are  descendants  of  reptiles  
(iv) Eg2
...
 Taxonomic  classification  systems  
(i) Focus  on  evolution  vs
...
 less  emphasis  
(iii) Uses  DNA  and  RNA  sequencing  vs
...
Manual  creations  of  such  diagrams  would  be  difficult  with  many  species  

(v) No   distinction   between   extinct   and   extant   (surviving)   species   (both   may   be  
included  in  cladograms)  vs
...
 monkeys,  apes  and  
humans  have  opposable  thumbs  =  primate  
(c) Unique  features  may  determine  a  species  but  useless  for  determining  ancestry  
(i) Only  shared  by  virtue  of  having  long  histories  
(ii) Evolutionary  ‘hangers  on’  that  have  persisted  in  several  linages  and  continued  
to  diverge  so  are  no  longer  closely  related  eg
...
 In  agricultural  system  
(c) Advantageous  to  humans  
(d) Relatively  fast  
(e) Costs  money  
(3) Both  
(a) Involve  selection  of  parents  for  advantageous  traits  
(b) Involve  inheritance  of  alleles  
(c) Change  allele  frequencies  
(d) Takes  several  generations  
(4) Process  of  selectively  breeding  –  artificial  selection  
(a) Choose  a  male  and  female  with  the  desired  characteristic(s)  and  mate  them  

(b) Inspect  the  offspring  
(i) Any  traits  with  a  high  level  of  heritability  will  come  out  in  the  offspring  
(ii) Very  few  characteristics  are  the  result  of  one  gene  and  are  usually  a  result  of  
polygenes/epistasis/linkage  
(iii) Easy  to  identify  characteristics  that  affect  appearance  eg
...
 tameness    
(c) Allow  the  best  offspring  to  breed  
(d) Repeat  this  over  several  generations  
(e) Until  all  offspring  show  homozygosity  for  the  desired  characteristic  
 
xix) Describe   how   artificial   selection   has   been   used   to   produce   the   modern   dairy   cow   and   to  
produce  bread  wheat  (Triticum  aestivum)  (HSW6a,  6b)  
(1) Characteristics  artificially  selected  for  in  cows  
(a) Modern   dairy   cow   Chillingham   White   is   thought   to   be   the   breed   that   looks   the  
most  like  original  wild  cattle  
(b) Also  Holstein-­‐Friesian,  Brown  Swiss,  Guernsey,  Ayrshire,  Jersey,  Milky  Shorthorn  
(c) Some  breeds  of  dairy  cattle  have  thick  coats  and  can  live  in  the  Scottish  Highlands  
(d) High  milk  yield  
(e) High  meat  yield  
(f) Ability  to  survive  in  particular  environments  
(g) Docility  (tameness)  
(2) Principles  of  selecting  breeding  bulls  practiced  today  
(a) Progenies   of   the   bulls   are   tested   to   determine   which   ones   produce   high   milk-­‐
yielding  daughter  cows  (only  a  few  need  to  be  kept  to  inseminate  many  cows)  
(b) Milk  yield  from  each  cow  is  measured  and  recorded  
(c) Best  cows  can  be  given  hormones  to  encourage  prolific  egg  production  
(d) Cows  are  artificially  inseminated  with  semen  from  a  good  quality  bull  OR  eggs  are  
fertilised  in  vitro  
(e) Embryos  are  implanted  into  surrogate  mothers  
(f) Embryos  can  also  be  cloned  
(g) à  A  few  elite  cows  can  give  rise  to  more  offspring  than  they  would  naturally  
(3) Wheat    
(a) Can  grow  in  large  areas  of  the  world  
(b) Makes  up  33%  of  all  cereal  crops  
(c) Breeders   continue   to   carry   out   selection   programmes   to   produce   improved  
varieties  
(d) Characteristics  artificially  selected  for  in  wheat  
(i) Increased  yield  
(ii) High  protein  content  
(iii) Stiff  straw  (stem)  –  to  support  large  shaft  
(iv) Resistance   to   lodging   –   prevent   stems   falling   over   in   wind   and   rain   which  
would  make  harvest  more  difficult  

(v) Resistance  to  disease  eg
...
 wild  wheat  
(a) Large,  plump  seeds  vs
...
 seeds  can’t  disperse  themselves  
(d) Non-­‐shattering   rachis   (stem)   when   grape   is   ripe   vs
...
 hulls  (outer  coating/shell  of  seed)  
(i) Hull  around  seed  comes  off  during  threshing  
(ii) Grain  only  has  to  be  winnowed  to  blow  the  hull  away  from  the  seeds  
(iii) Wild   varieties   are   hulled   instead   –   hull   stays   around   the   seed   even   after   it   has  
been  threshed,  and  so  needs  to  be  pounded  
(iv) Laborious  and  breaks  the  grain  so  it  is  less  suitable  for  storage  
(f) Seeds  retained  and  germinated  together  vs

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